OK, I just tried it and I was able to play with someone who had the Steam version of the game, 3.03 (NSPY), who did not have the DLCs. If he hosted, then the DLCs were deactivated. If I hosted we could both use the DLCs. I think the other person has to have the Steam version of the game for this to work, as that seems to come with the necessary files for the DLC, even if you only bought the base game.